Heating assembly comprising a ptc element, in particular for a motor vehicle

ABSTRACT

The invention relates to a heating assembly comprising at least one PTC element, in particular for a motor vehicle. According to the invention, the PTC element ( 2 ) is positioned between metal sheets ( 3, 4 ), which are used to make electrical contact, said sheets ( 3, 4 ) and the PTC element ( 2 ) are bonded by means of an adhesive ( 5 ) and the adhesive ( 5 ) has a minimum specific electrical resistance of 50 ohms×cm and a maximum specific electrical resistance of 500 ohms×cm. In addition, solder can be used as an alternative to the adhesive ( 5 ).

The invention relates to a heating assembly with a PTC element, inparticular for a motor vehicle, according to the precharacterizingclause of claim 1.

DE 101 44 757 A1 discloses a heating assembly with a PTC element forpassenger vehicles, a supplementary heating system being provided,having a heating element through which heating air flows duringoperation of the supplementary heating system and having at least oneair outlet opening in the foot region of a passenger compartment, towhich the heating air is conducted. To allow a vertical temperaturestratification that is also comfortable in particular for seats at therear to be produced in the passenger compartment in a flexible way, theheating element takes the form of an electrical PTC element, which isarranged directly at the air outlet opening in the foot region. Asupplementary heater of this type still leaves something to be desired.According to one disclosed exemplary embodiment, a PTC element in theform of a number of heating honeycombs is arranged in a plastic frame(not described in any more detail), which surrounds the air outletopening.

The attachment of contact plates to PTC elements conventionally takesplace by means of an adhesive, insulating adhesives with a resistivityof over 10,000 ohms×cm or conducting adhesives with a resistivity ofunder 10 ohms×cm being used—depending on the application. Such bondsbetween the contact plate and the PTC element leave something to bedesired.

The object of the invention is to provide an improved heating assemblywith a PTC element.

This object is achieved by a heating assembly with a PTC element havingthe features of claim 1. Advantageous refinements are the subject of thesubclaims.

According to the invention, a heating assembly with at least one PTCelement is provided, the PTC element being arranged between contactplates which serve for making electrical connection, the contact platesand the PTC element being bonded by means of an adhesive which has aresistivity of at least 50 ohms×cm and at most 500 ohms×cm, preferablyof at least 80 ohms×cm and at most 150 ohms×cm, in particular of 100ohms×cm+/−10%.

Allowance for heating and safety aspects is made at the same time if anadhesive with a certain resistivity, in particular an adhesive with aresistivity of 50 to 500 ohms×cm, is chosen, the PTC elements beingbonded to contact plates by this electrically conducting adhesive. Theresistivity is to be chosen here on the one hand such that the risk of ashort-circuit between the contact plates is avoided. On the other hand,the resistivity is to be chosen such that, in the event of the heatingassembly being damaged as a consequence of aging, the adhesive layer canundergo enforced relaxation, averting direct contact between the PTCelement and the contact plates, but the adhesive layer having adequateelectrical conductivity to maintain the heating function should thisoccur. In this case, the adhesive layer has an additional electricalresistance and displays acceptable heating output. The advantage of theinvention is that in this case the heating function is not disturbed.The aforementioned range for the resistivity has been found to be thatwhich is most suitable.

It is preferred for the layer thickness of the adhesive between the PTCelement and a contact plate before enforced relaxation to be negligibleand after enforced relaxation to be at most 0.02 μm, in particular 0.01μm+/−10%.

Instead of an adhesive, a solder with a corresponding resistivity mayalso be used.

The invention is explained in detail below on the basis of an exemplaryembodiment with reference to the drawing, in which:

FIG. 1 shows a view of a heating assembly according to the invention,

FIG. 2 shows an enlarged section through the heating assembly from FIG.1 along the line II-II in FIG. 1,

FIG. 3 shows a section along the line III-III in FIG. 2,

FIG. 4 shows a representation corresponding to FIG. 2 after enforcedrelaxation,

FIG. 5 shows an equivalent circuit diagram which illustrates theresistances, and

FIG. 6 shows an equivalent diagram corresponding to FIG. 5 whichillustrates the resistances after enforced relaxation.

In the case of a PTC heating assembly 1 with ceramic PTC elements, a PTCelement 2 is respectively bonded in place between two contact plates 3(positive terminal) and 4 (negative terminal) that are arranged parallelto each other, in the present case by means of an adhesive 5 which has aresistivity of approximately 100 ohms×cm. Corrugated ribs 6 are attachedby means of a corresponding adhesive bond on those sides of the contactplates 3 and 4 which in each case lie opposite the PTC element 2, and arespective further contact plate 3 and 4 is attached to said corrugatedribs, likewise by means of a corresponding adhesive bond. Arrangedaround the assembly described above is a plastic frame 7. The flowdirection of the air is in the viewing direction in the case of FIG. 1.

Hereafter, the length of a PTC element 2 is denoted by L, in the presentcase 035 mm, the width of the PTC element 2 by, in the present case 8mm, and the thickness of the PTC element 2 by e, in the present case 1.4mm. The average thickness of the adhesive layer between the surface ofthe PTC element 2 and a contact plate 3 or 4 is denoted by eta, in thepresent case 0.01 μm. The width of the adhesive around the PTC element 2between the contact plates 3 and 4 is denoted by s and in the presentcase is 1 mm.

The voltage between the contact plates 3 and 4 is denoted by U and,according to the present exemplary embodiment, is 13 V, but may also begreater, for example 48 V.

The resistivity of the adhesive 5 is denoted by Rho_(adhes), theresistance of the PTC element 2 by R_(PTC), the resistance of theadhesive layer between the PTC element 2 and the contact plate 3 or 4 byR_(adhes).

The output of the PTC element 2 is denoted by P_(PTC), the output of theadhesive 5 between the contact plates 3 and 4 by P_(b), the output ofthe PTC element 2 in connection with the adhesive 5 without enforcedrelaxation by P_(adhesion+PTC without enforced relaxation), the outputof the PTC element 2 in connection with the adhesive 5 with enforcedrelaxation by P_(adhesion+PTC after enforced relaxation), the overalloutput by P_(total).

Here, the resistance R_(adhes) of the adhesive 5 between the PTC elementand a contact plate 3 or 4 and the resistance R_(b) of the adhesive 5surrounding the PTC element 2 is obtained as follows:R _(adhes) =Rho _(adhes) ×eta/(L×l)   (1)R _(b) =Rho _(adhes) ×e/(2(L+l)×s)   (2)

Equivalent diagrams for the circuits of the resistances are representedin FIGS. 5 and 6.

The output P_(adhesion+PTC without enforced relaxation) and the outputP_(adhesion+PTC after enforced relaxation) are obtained byP _(adhesion+PTC without enforced relaxation) =U ² /R _(PTC)   (3)P _(adhesion+PTC after enforced relaxation) =U ²/(2R _(adhes) +R _(PTC))  (4)

To bring about optimum efficiency of the PTC element 2, the ratioP_(PTC)/P_(b) should be chosen to be as great as possible. Furthermore,however, the ratio ofP_(adhesion+PTC without enforced relaxation)/P_(adhesion+PTC after enforced relaxation)should lie as close as possible to 1. In this case, the ratio ofP_(PTC)/P_(b) lies in particular between about 4 and 40 and the ratio ofP_(adhesion+PTC without enforced relaxation)/P_(adhesion+PTC after enforced relaxation)lies between about 1.2 and 1.02.

LIST OF DESIGNATIONS

-   1 heating assembly-   2 PTC element-   3 contact plate-   4 contact plate-   5 adhesive-   6 corrugated rib-   7 plastic frame

1. A heating assembly with at least one PTC element, in particular for amotor vehicle, the PTC element being arranged between contact plateswhich serve for making electrical connection, the contact plates and thePTC element being bonded by means of an adhesive wherein the adhesivehas a resistivity of at least 50 ohms×cm and at most 500 ohms×cm.
 2. Theheating assembly as claimed in claim 1, wherein the adhesive has aresistivity of at least 80 ohms×cm and at most 150 ohms×cm, inparticular of 100 ohms×cm+/−10%.
 3. The heating assembly as claimed inclaim 1, wherein the layer thickness of the adhesive between the PTCelement and a contact plate before enforced relaxation is negligible andafter enforced relaxation is at most 0.02 μm, in particular 0.01μm+/−10%.
 4. A heating assembly with at least one PTC element, inparticular for a motor vehicle, the PTC element being arranged betweencontact plates which serve for making electrical connection, the contactplates and the PTC element being bonded by means of a solder, whereinthe solder has a resistivity of at least 50 ohms×cm and at most 500ohms×cm.
 5. The heating assembly as claimed in claim 4, wherein thesolder has a resistivity of at least 80 ohms×cm and at most 150 ohms×cm,in particular of 100 ohms×cm+/−10%.
 6. The heating assembly as claimedin claim 4, wherein the layer thickness of the solder between the PTCelement and a contact plate before enforced relaxation is negligible andafter enforced relaxation is at most 0.02 μm, in particular 0.01μm+/−10%.
 7. An adhesive or a solder for bonding between a ceramic PTCelement and an electrically conducting contact plate characterized inthat the adhesive (5) or the solder has a resistivity of at least 50ohms×cm and at most 500 ohms×cm.
 8. The heating assembly as claimed inclaim 2, wherein the layer thickness of the adhesive between the PTCelement and a contact plate before enforced relaxation is negligible andafter enforced relaxation is at most 0.02 μm, in particular 0.01 μm±10%.9. The heating assembly as claimed in claim 5, wherein the layerthickness of the solder between the PTC element and a contact platebefore enforced relaxation is negligible and after enforced relaxationis at most 0.02 μm, in particular 0.01 μm±10%.